1. Field of the Invention
The present invention relates generally to torque control arrangements for four-wheel-drive vehicles, and more particularly, to an arrangement for controlling torque transmitted from a power plant through a torque transmitting path to front and rear wheels in a four-wheel-drive vehicle.
2. Description of the Prior Art
One type of four-wheel drive vehicle in which an all-wheel drive and a front or rear wheel drive are adopted selectively in response to the conditions of roadways has been put to practical use. In such a type of four-wheel-drive vehicle, for the purpose of having a smooth changeover of the all-wheel drive into the front or rear wheel drive and also a smooth changeover of the front or rear wheel drive into the all-wheel drive, there has been proposed to use a torque controlling arrangement including a wet clutch device disposed in either a front torque transmitting path through which torque from a power plant is transmitted to front wheels or a rear torque transmitting path through which the torque from the power plant is transmitted to rear wheels, as disclosed in, for example, the Japanese utility model application published before examination under the publication number 56/122630.
In a four-wheel-drive vehicle which employs the torque controlling arrangement provided with the wet clutch device as mentioned above, the changeover between the all-wheel drive and the front or rear wheel drive is carried out in accordance with manipulation of a switch or the like provided on, for example, a gear shift lever conducted by a driver of the vehicle in response to the driving condition of the vehicle, the conditions of roadways and so on. Accordingly, it is required for the driver to have annoying handlings whenever the changeover between the all-wheel drive and the front or rear wheel drive is carried out, and further there is the defect that undesirable shock is apt to be induced on the occasion of the changeover between the all-wheel drive and the front or rear wheel drive in the case where a speed difference between the front and rear wheels is relatively large.
So, there has been also proposed a torque control arrangement for a four-wheel-drive vehicle in which a viscous fluid clutch which is operative to vary torque transmitted therethrough in response to a difference between input speed and output speed thereof is disposed in one of torque transmitting paths provided on the side of right and left front wheels and on the side of right and left rear wheels, respectively, and thereby torque transmitted to the right and left front wheels or the right and left rear wheels is varied in response to a difference between the average speed relative to the right and left front wheels and the average speed relative to the right and left rear wheels, so that the changeover of the all-wheel drive into the front or rear wheel drive and the changeover of the front or rear wheel drive into the all-wheel drive are performed automatically.
However, in such a torque control arrangement provided with the viscous fluid clutch for varying the torque transmitted to the front wheels or the rear wheels in response to the speed difference between the front and rear wheels, there are problems arising as follows.
In the torque control arrangement provided with the viscous fluid clutch as mentioned above, even if a difference between input speed and output speed of the viscous fluid clutch is reduced enough to be close to zero under a condiction wherein a vehicle employing the arrangement is making a stabilized straight drive, a certain small torque is actually transmitted through the viscous fluid clutch and thereby generation of some heat is raised in the viscous fluid clutch. Such a generation of heat exerts a harmful influence upon the viscous fluid clutch so as to lower the durability of the same.
Further, considering an ordinary case where a viscous fluid clutch is disposed in a torque transmitting path in a four-wheel-drive vehicle provided on the side of front wheels each having the same diameter as each of rear wheels, by way of example, such a torque transmission characteristic as shown in FIG. 1, where the axis of abscissas represents speed difference (.DELTA.n) between input speed and output speed of a fluid torque transmitting device, such as the viscous fluid clutch, obtained by subtracting the output speed from the input speed and the axis of ordinates represents torque (T) transmitted to front or rear wheels of a four-wheel-drive vehicle in which the fluid torque transmitting device is employed, is obtained through the viscous fluid clutch. That is, positive torque (referred to as driving torque, hereinafter) is transmitted through the viscous fluid clutch to the rear wheels when the speed difference .DELTA.n is positive, as indicated with a curve of solid line in FIG. 1, while negative torque (referred to as braking torque, hereinafter) is transmitted through the viscous fluid clutch to the rear wheels when the speed difference .DELTA.n is negative, as indicated with a curve of dot-and-dash line in FIG. 1. In the situation wherein the braking torque is transmitted to the rear wheels in this manner, the rear wheels bring forth undesirable running resistance to the vehicle, and this results in higher fuel consumption and increased wear to various parts of the vehicle.